Add missing `mutate()` in `AtomicOperation`

It seems [AtomicOperation.mutate()](https://docs.deno.com/api/deno/~/Deno.AtomicOperation#method_mutate_0) is missing in [AtomicOperation](https://github.com/denoland/denokv/blob/5697a1b00783c5b9f234319ee4d995e387772c14/npm/src/kv_types.ts#L494).